Fractions and decimals are two different systems in Finnish, and each hides a trap for English speakers. Fractions are built transparently from ordinals plus the word osa ("part") — once you know the ordinals, you know the fractions. Decimals hide a purely orthographic surprise: Finnish writes the decimal separator as a comma, not a point, and reads it aloud as pilkku. Behind both sits the same quiet rule that governs all Finnish counting — a measured amount goes into the partitive, so "half a litre" is puoli litraa, not *puoli litra. The ordinals these fractions lean on are on Ordinal Numbers.
Fractions: ordinal + osa
The workhorse pattern is dead simple: take the ordinal and glue osa ("part") onto it, written as one word. A third is literally "a third part":
- kolmasosa = 1/3 (kolmas
- osa)
- neljäsosa = 1/4
- viidesosa = 1/5
- kymmenesosa = 1/10
- sadasosa = 1/100 (sadas, hundredth, + osa)
The one you cannot build this way is 1/2, which has its own word, puoli ("half"). English does the same — we say half, not *twoth — so this exception should feel familiar.
Söin kolmasosan kakusta.
I ate a third of the cake.
Vain kymmenesosa hakijoista pääsi sisään.
Only a tenth of the applicants got in.
Counting whole fractions: the number takes over the case
To say two thirds or three quarters, put a cardinal in front of the fraction — and now the ordinary Finnish counting rule kicks in. Any number above one forces what follows into the partitive singular, so the osa becomes osaa:
- kaksi kolmasosaa = 2/3 ("two thirds")
- kolme neljäsosaa = 3/4 ("three quarters")
Kaksi kolmasosaa suomalaisista asuu kaupungeissa.
Two thirds of Finns live in cities.
Kolme neljäsosaa työstä on jo tehty.
Three quarters of the work is already done.
This is the same mechanism that turns yksi euro into kaksi euroa — the number governs the case of the noun. It is spelled out in full on The Partitive After Numbers.
The "of" complement: elative
When you say a fraction of something, Finnish marks that "something" with the elative case, -sta / -stä ("out of"). It is the natural image: a third taken out of the cake.
Kolmasosa kakusta on jo syöty.
A third of the cake has already been eaten.
Puolet ryhmästä oli myöhässä.
Half of the group was late.
Watch the consonant gradation on the noun: kakku → kaku- → kakusta (the double k softens to single k before the ending). Note also that "half of a group/set" is puolet (a plural-looking form), not puoli — puoli is "half of one thing", puolet is "half of a collection": puolet opiskelijoista ("half of the students").
Measured amounts: puoli + partitive
The measurement pattern is where English speakers slip. Half an hour, half a litre, half a kilo — in each, the measured unit goes into the partitive singular after puoli:
- puoli tuntia = half an hour (tunti → tuntia)
- puoli litraa = half a litre (litra → litraa)
- puoli kiloa = half a kilo (kilo → kiloa)
Anna minulle puoli kiloa jauhelihaa.
Give me half a kilo of minced meat.
Odotin bussia puoli tuntia sateessa.
I waited half an hour for the bus in the rain.
Matka kestää puolitoista tuntia.
The trip takes an hour and a half.
Decimals: the comma is read pilkku
Now the orthographic trap. In Finnish, the decimal separator is a comma, and it is read aloud as pilkku ("comma"). Where English writes and says "three point one four," Finnish writes 3,14 and says kolme pilkku yksi neljä. The digits after the comma are read individually, exactly as in English — it is only the separator that changes.
| Written | Read aloud | English |
|---|---|---|
| 3,14 | kolme pilkku yksi neljä | three point one four |
| 0,5 | nolla pilkku viisi | zero point five |
| 2,50 | kaksi pilkku viisi nolla | two point five zero |
Piin likiarvo on kolme pilkku yksi neljä.
Pi is approximately three point one four (3.14).
Lämpötila nousi vain nolla pilkku viisi astetta.
The temperature rose only half a degree (0.5).
Percentages: prosenttia
A percentage uses prosentti ("percent," symbol %), and — being counted by a number — it appears in the partitive prosenttia after any figure. In writing, Finnish puts a space before the % sign (10 %).
Vain kymmenen prosenttia äänesti vaaleissa.
Only ten percent voted in the election.
Hinnat nousivat kaksikymmentäviisi prosenttia vuodessa.
Prices rose twenty-five percent in a year.
Common Mistakes
1. Writing or reading a decimal point.
❌ Piin arvo on kolme piste yksi neljä.
Incorrect — the separator is a comma, read 'pilkku', not 'piste'.
✅ Piin arvo on kolme pilkku yksi neljä.
Pi is three point one four (3,14).
2. Leaving the measured unit in the nominative after puoli.
❌ Odotin puoli tunti.
Incorrect — the amount takes the partitive.
✅ Odotin puoli tuntia.
I waited half an hour.
3. Using the wrong case for the "of" complement.
❌ Söin kolmasosan kakku.
Incorrect — 'of the cake' needs the elative.
✅ Söin kolmasosan kakusta.
I ate a third of the cake.
4. Forgetting the partitive after a counting number in front of a fraction.
❌ kaksi kolmasosa
Incorrect — the number forces the partitive on osa.
✅ kaksi kolmasosaa
two thirds
Key Takeaways
- Fractions = ordinal + osa (kolmasosa), with puoli for 1/2 and the formal -nnes variant (neljännes).
- A number before a fraction forces the partitive (kaksi kolmasosaa).
- "Of" something = elative (kolmasosa kakusta); "half of a group" is puolet
- elative.
- Measured amounts after puoli / puolitoista take the partitive (puoli tuntia).
- Decimals use a comma read pilkku (3,14 = kolme pilkku yksi neljä) — never "point."
- Percentages: prosenttia in the partitive after a figure.
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Start learning Finnish→Related Topics
- Money, Weights and MeasuresA2 — Prices and quantities follow the counting rule: after a number the unit is partitive singular — viisi euroa, kaksi kiloa, litra maitoa. Watch harmony and length.
- Numbers Take the Partitive Singular: kaksi kissaaA2 — The signature Finnish counting rule — every cardinal above one governs the partitive singular of the counted noun, which stays singular: kaksi kissaa, sata euroa.
- Writing Numbers, Dates and DecimalsA2 — How Finnish writes figures: decimal comma, spaced thousands, day-first dates, the period on ordinals, and the colon that glues a case ending onto a digit.
- Cardinal Numbers 1–10A1 — The base ten — yksi, kaksi … kymmenen — with their inflection stems, and the rule that a number above one takes the counted noun in the partitive singular.